You have run into two limitations in Word.
The first limitation is that the online version isn't very stable when it comes to numbering. It will be safer to edit documents with numbering in the full, locally installed version of Word on a PC or Mac computer.
The second limitation is with numbering itself. The safest way to restart numbering in Word is to set up a multilevel list so that higher levels can restart lower ones. Also, numbering levels should be associated with paragraph styles. The general principles are discussed in the article at https://shaunakelly.com/word/numbering/numbering20072010.html.